I want to spend my 21st birthday on a trip to either Las Vegas or Los Angeles, but my birthday is on Christmas Eve, and I prefer to not celebrate it around Christmas. So I was thinking of planning the trip around New Year instead, but my only concern is the weather and the activities. I generally enjoy taking vacations during the summer since the weather is warmer and there are usually a lot more attractions open, but I know those places can get ridiculously hot (I get easily sick in the heat) and by the time summer rolls around, the excitement of being 21 would be gone, but I don't want to go on a trip to somewhere so huge and expensive on a gloomy and cold day. A lot of things might be closed for the holidays too, though I'm unsure. I also heard that both cities are sort of equal when it comes to attractions, though I'm tied between them since-after researching- Las Vegas looks like it has more to do, but Los Angeles looks fun as well, and is somewhere I've wanted to visit for years now since I work in the entertainment industry (Specifically remote opportunities from Los Angeles as well writing a lot of stories that take place in that location) and have some sentimental connections with the place. Any suggestions on the best time of year to go, and which location is better around those times?

Oh, and in case anyone was wondering, my 21st birthday won't be for a while now, I just wanted to start planning/saving now since I know it will be an expensive trip and want a general idea of what I might do so I can start making financial goals.